diff --git a/CHANGES/928.bugfix.rst b/CHANGES/928.bugfix.rst
new file mode 100644
index 000000000..94f410a41
--- /dev/null
+++ b/CHANGES/928.bugfix.rst
@@ -0,0 +1,3 @@
+Covered the unreachable code path in
+``multidict._multidict_base._abc_itemsview_register()``
+with typing -- by :user:`skinnyBat`.
diff --git a/CHANGES/928.contrib.rst b/CHANGES/928.contrib.rst
new file mode 100644
index 000000000..697e38edf
--- /dev/null
+++ b/CHANGES/928.contrib.rst
@@ -0,0 +1,3 @@
+Added tests to have full code coverage of the
+``multidict._multidict_base._viewbaseset_richcmp()`` function
+-- by :user:`skinnyBat`.
diff --git a/multidict/_multidict_base.py b/multidict/_multidict_base.py
index 394466548..de2f762a5 100644
--- a/multidict/_multidict_base.py
+++ b/multidict/_multidict_base.py
@@ -1,5 +1,11 @@
+import sys
 from collections.abc import ItemsView, Iterable, KeysView, Set, ValuesView
 
+if sys.version_info >= (3, 11):
+    from typing import assert_never
+else:
+    from typing_extensions import assert_never
+
 
 def _abc_itemsview_register(view_cls):
     ItemsView.register(view_cls)
@@ -46,6 +52,8 @@ def _viewbaseset_richcmp(view, other, op):
             if elem not in view:
                 return False
         return True
+    else:  # pragma: no cover
+        assert_never(op)
 
 
 def _viewbaseset_and(view, other):
diff --git a/setup.cfg b/setup.cfg
index 323ea4ba0..a37ebfef5 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -43,6 +43,8 @@ classifiers =
 
 [options]
 python_requires = >= 3.7
+install_requires =
+  typing-extensions >= 4.1.0; python_version < '3.11'
 packages =
   multidict
 
diff --git a/tests/test_multidict.py b/tests/test_multidict.py
index 3173fe24c..bcfa699c1 100644
--- a/tests/test_multidict.py
+++ b/tests/test_multidict.py
@@ -286,10 +286,25 @@ def test_keys_is_set_less(self, cls: Type[MutableMultiMapping[str]]) -> None:
 
         assert d.keys() < {"key", "key2"}
 
-    def test_keys_is_set_less_equal(self, cls: Type[MutableMultiMapping[str]]) -> None:
-        d = cls([("key", "value1")])
+    @pytest.mark.parametrize(
+        ("contents", "expected"),
+        (
+            ([("key", "value1")], True),
+            ([("key", "value1"), ("key2", "value2")], True),
+            ([("key", "value1"), ("key2", "value2"), ("key3", "value3")], False),
+            ([("key", "value1"), ("key3", "value3")], False),
+        ),
+    )
+    def test_keys_is_set_less_equal(
+        self,
+        cls: Type[MutableMultiMapping[str]],
+        contents: List[Tuple[str, str]],
+        expected: bool,
+    ) -> None:
+        d = cls(contents)
 
-        assert d.keys() <= {"key"}
+        result = d.keys() <= {"key", "key2"}
+        assert result is expected
 
     def test_keys_is_set_equal(self, cls: Type[MutableMultiMapping[str]]) -> None:
         d = cls([("key", "value1")])
@@ -297,23 +312,95 @@ def test_keys_is_set_equal(self, cls: Type[MutableMultiMapping[str]]) -> None:
         assert d.keys() == {"key"}
 
     def test_keys_is_set_greater(self, cls: Type[MutableMultiMapping[str]]) -> None:
-        d = cls([("key", "value1")])
+        d = cls([("key", "value1"), ("key2", "value2")])
 
-        assert {"key", "key2"} > d.keys()
+        assert d.keys() > {"key"}
 
+    @pytest.mark.parametrize(
+        ("set_", "expected"),
+        (
+            ({"key"}, True),
+            ({"key", "key2"}, True),
+            ({"key", "key2", "key3"}, False),
+            ({"key3"}, False),
+        ),
+    )
     def test_keys_is_set_greater_equal(
-        self,
-        cls: Type[MutableMultiMapping[str]],
+        self, cls: Type[MutableMultiMapping[str]], set_: Set[str], expected: bool
+    ) -> None:
+        d = cls([("key", "value1"), ("key2", "value2")])
+
+        result = d.keys() >= set_
+        assert result is expected
+
+    def test_keys_less_than_not_implemented(
+        self, cls: Type[MutableMultiMapping[str]]
+    ) -> None:
+        d = cls([("key", "value1")])
+
+        sentinel_operation_result = object()
+
+        class RightOperand:
+            def __gt__(self, other: KeysView[str]) -> object:
+                assert isinstance(other, KeysView)
+                return sentinel_operation_result
+
+        assert (d.keys() < RightOperand()) is sentinel_operation_result
+
+    def test_keys_less_than_or_equal_not_implemented(
+        self, cls: Type[MutableMultiMapping[str]]
     ) -> None:
         d = cls([("key", "value1")])
 
-        assert {"key"} >= d.keys()
+        sentinel_operation_result = object()
+
+        class RightOperand:
+            def __ge__(self, other: KeysView[str]) -> object:
+                assert isinstance(other, KeysView)
+                return sentinel_operation_result
+
+        assert (d.keys() <= RightOperand()) is sentinel_operation_result
+
+    def test_keys_greater_than_not_implemented(
+        self, cls: Type[MutableMultiMapping[str]]
+    ) -> None:
+        d = cls([("key", "value1")])
+
+        sentinel_operation_result = object()
+
+        class RightOperand:
+            def __lt__(self, other: KeysView[str]) -> object:
+                assert isinstance(other, KeysView)
+                return sentinel_operation_result
+
+        assert (d.keys() > RightOperand()) is sentinel_operation_result
+
+    def test_keys_greater_than_or_equal_not_implemented(
+        self, cls: Type[MutableMultiMapping[str]]
+    ) -> None:
+        d = cls([("key", "value1")])
+
+        sentinel_operation_result = object()
+
+        class RightOperand:
+            def __le__(self, other: KeysView[str]) -> object:
+                assert isinstance(other, KeysView)
+                return sentinel_operation_result
+
+        assert (d.keys() >= RightOperand()) is sentinel_operation_result
 
     def test_keys_is_set_not_equal(self, cls: Type[MutableMultiMapping[str]]) -> None:
         d = cls([("key", "value1")])
 
         assert d.keys() != {"key2"}
 
+    def test_keys_not_equal_unrelated_type(
+        self, cls: Type[MutableMultiMapping[str]]
+    ) -> None:
+        d = cls([("key", "value1")])
+
+        assert d.keys() != "other"
+
     def test_eq(self, cls: Type[MutableMultiMapping[str]]) -> None:
         d = cls([("key", "value1")])